Corrosion is a major concern with underwater transmission cables, as the underwater environment is deleterious to cables in many ways. The cables are often exposed to collision with commercial fishing gear and other apparatus. Collision with fishing gear may destroy the cables or cause nicks or punctures in the cable outer jackets, exposing the inner transmission wires to sea water. The sea water environment is particularly corrosive due to its potential to interact with the metallic surface of transmission cables and produce hydrogen or hydroxyl ions, creating concern for hydrogen-induced attenuation of the cable or inner fibers. In the sea water environment, there is also a current-induced corrosion factor, i.e., the ocean current acts as an electrical current which, upon passing through the earth's magnetic field, produces a Hall effect to create a potential field along a cable and cause accelerated corrosion. Abrasion caused by wave and current action can further be a source of corrosion.
Traditionally, underwater cables have been strengthened and protected by layers of armor or jackets surrounding the core that contains communications and/or power transmission media. The use of materials considered to be corrosion-resistant has also been employed to address the impact of the sea environment. Stainless steel alloys, for example, have commonly been considered a preferred corrosion resistant material for use in protecting underwater transmission media.
To illustrate, FIGS. 1 and 2 show a traditional deep water trunk power and communications transmission system 90. FIG. 1 shows a cross-sectional view along the line 10--10 of FIG. 2, which shows a perspective side view of the cable, with the layers exposed. A fiber optic core structure 1 is centrally disposed in the system. Referring to FIG. 2, the core structure 1 comprises an optical fiber 2, buffer coating 3, and buffer jacket 4. As seen in FIGS. 1 and 2, the core structure 1 is surrounded with a plurality of steel strands 5 which are selected and arranged to densely pack a circular cross-section surrounding the fiber optic core structure 1. The fiber optic core structure 1 and steel strands 5 serve, respectively, as communications transmission media and power transmission media. The steel strands also serve to strengthen and protect the cable. A welded swaged copper tube 6 surrounds the steel strands, providing a pressure barrier to absorb and more uniformly distribute underwater pressures to protect the core structure 1 from damage. The copper tube 6 may also serve to balance torque by resisting any torque that is imposed upon the cable by the steel strands. Insulation 7, an electrical and mechanical shield 8, and a protective jacket 9 form successive cylindrical layers surrounding the welded swaged copper tube 6.
Such prior art underwater cables utilizing steel strength systems have several disadvantages, however. Steel has lower electrical conductivity than other materials that may be used for power or communications media such as, for example, copper. Thus, to achieve the same conductivity as when other transmission materials (e.g., copper) are used, the steel strands used as a transmission medium must have a larger cross section which adversely impacts on the weight and rigidity of the cables being used. Steel strands used as a power transmission or communications transmission medium also must be electrically insulated and corrosion protected throughout the cable system.
Other disadvantages of using steel in systems for transmission cables result from the weight and rigidity of steel. Machinery used to manufacture cable employing steel must be capable of handling the additional weight and rigidity of steel as compared to the weight and rigidity of other power transmission and communication transmission cable components and transmission media. Weight is a disadvantage when power transmission and communications transmission cables are tethered to underwater vehicles or supported from buoys. The rigidity of cable components increases the minimum permissible radius of bend which a cable can achieve.
Additionally, while steel may be more corrosion-resistant than other materials known for use in transmission media, steel will corrode when exposed to sea water. Corrosion can weaken the steel and shorten the useful life of the cable. Hydrogen and other products generated by the corrosion of steel can adversely affect the light transmissivity of glass fiber used as a communications transmissions medium thereby degrading its performance. While it generally is known to cover with various protective jackets the steel strands used in underwater cables, the potential for puncture of the jackets is always present.
To address drawbacks involved with the use of steel, attempts have been made to fabricate cables comprised of other materials. For example, certain complex nickel alloys have excellent corrosion resistance are compatible for use in underwater cables, but their use has been limited due to the great expense involved with these alloys. Aluminum has been used to address, for example, weight and rigidity factors, but it is low in strength as compared with steel and subject to corrosion. Synthetic materials require new cable designs and new methods for terminating the cable.
With regard to terminating the cable, steel transmission cables are conventionally terminated in a socket and cone configuration, as depicted in FIGS. 3 through 5. The steel strands 5 are arranged around the inner wall 18 of a conical socket 20. A mating cone 30 is placed over the steel strands 5, inserted within socket 20, and pressed into place to form the completed termination, shown in FIG. 5. The process of terminating the steel-type transmission cable generally is very time consuming and requires the use of heavy and expensive hydraulic equipment.
Accordingly, there is a need for lighter weight, corrosion resistant strength systems for transmission cable and new methods for terminating the cables. The present invention provides a non-metallic, corrosion resistant strength system for underwater power transmission and communications transmission cables which permits higher conductivity per cable size, avoids corrosive products that decrease the light transmissivity of the optical fiber, and provides lower costs of electrical insulation, water protection, and mechanical termination.